Recycling backlights with semi-specular components

ABSTRACT

A hollow light-recycling backlight has a “semi-specular” component providing a balance of specularly and diffusely reflected light improving the uniformity of the light output. The component may be arranged on the reflectors ( 1021 ), ( 1014 ) or inside the cavity ( 1016 ). This balance is achieved by designing the component&#39;s “transport ratio” defined by (F−B)/(F+B), (F and B are the amounts of incident light scattered forwards and backwards respectively by the component in the plane of the cavity) to lie in a certain range. Furthermore, the product of the front and back reflector “hemispherical” reflectivities should also lie in a given range. Alternatively, the “cavity transport value”, a measure of how well the cavity can spread injected light from the injection point to distant points in the cavity should lie in a further range and the “hemispherical” reflectivity of the back reflector should be &gt;0.7.

RELATED APPLICATIONS

The following co-owned and copending PCT Patent Applications areincorporated herein by reference: PCT Patent Application No. ______,entitled BACKLIGHT AND DISPLAY SYSTEM USING SAME (Attorney Docket No.63274WO004); PCT Patent Application No. ______, entitled THIN HOLLOWBACKLIGHTS WITH BENEFICIAL DESIGN CHARACTERISTICS (Attorney Docket No.63031WO003); PCT Patent Application No. ______, entitled WHITE LIGHTBACKLIGHTS AND THE LIKE WITH EFFICIENT UTILIZATION OF COLORED LEDSOURCES (Attorney Docket No. 63033WO004); and PCT Patent Application No.______, entitled COLLIMATING LIGHT INJECTORS FOR EDGE-LIT BACKLIGHTS(Attorney Docket No. 63034WO004).

FIELD

The present invention relates to extended area light sources suitablefor illuminating a display or other graphic from behind, commonlyreferred to as backlights, as well as similar extended area lightingdevices. The invention is particularly applicable to backlights thatinclude a front and back reflector, between which is formed a hollowlight recycling cavity.

BACKGROUND

Backlights can be considered to fall into one of two categoriesdepending on where the internal light sources are positioned relative tothe output area of the backlight, where the backlight “output area”corresponds to the viewable area or region of the display device. The“output area” of a backlight is sometimes referred to herein as an“output region” or “output surface” to distinguish between the region orsurface itself and the area (the numerical quantity having units ofsquare meters, square millimeters, square inches, or the like) of thatregion or surface.

The first category is “edge-lit.” In an edge-lit backlight, one or morelight sources are disposed—from a plan-view perspective—along an outerborder or periphery of the backlight construction, generally outside thearea or zone corresponding to the output area. Often, the lightsource(s) are shielded from view by a frame or bezel that borders theoutput area of the backlight. The light source(s) typically emit lightinto a component referred to as a “light guide,” particularly in caseswhere a very thin profile backlight is desired, as in laptop computerdisplays. The light guide is a clear, solid, and relatively thin platewhose length and width dimensions are on the order of the backlightoutput area. The light guide uses total internal reflection (TIR) totransport or guide light from the edge-mounted lamps across the entirelength or width of the light guide to the opposite edge of thebacklight, and a non-uniform pattern of localized extraction structuresis provided on a surface of the light guide to redirect some of thisguided light out of the light guide toward the output area of thebacklight. Such backlights typically also include light managementfilms, such as a reflective material disposed behind or below the lightguide, and a reflective polarizing film and prismatic BEF film(s)disposed in front of or above the light guide to increase on-axisbrightness.

In the view of Applicants, drawbacks or limitations of existing edge-litbacklights include the relatively large mass or weight associated withthe light guide, particularly for larger backlight sizes; the need touse components that are non-interchangeable from one backlight toanother, since light guides must be injection molded or otherwisefabricated for a specific backlight size and for a specific sourceconfiguration; the need to use components that require substantialspatial non-uniformities from one position in the backlight to another,as with existing extraction structure patterns; and, as backlight sizesincrease, increased difficulty in providing adequate illumination due tolimited space or “real estate” along the edge of the display, since theratio of the circumference to the area of a rectangle decreases linearly(1/L) with the characteristic in-plane dimension L (e.g., length, orwidth, or diagonal measure of the output region of the backlight, for agiven aspect ratio rectangle).

The second category is “direct-lit.” In a direct-lit backlight, one ormore light sources are disposed—from a plan-viewperspective—substantially within the area or zone corresponding to theoutput area, normally in a regular array or pattern within the zone.Alternatively, one can say that the light source(s) in a direct-litbacklight are disposed directly behind the output area of the backlight.A strongly diffusing plate is typically mounted above the light sourcesto spread light over the output area. Again, light management films,such as a reflective polarizer film and prismatic BEF film(s), can alsobe placed atop the diffuser plate for improved on-axis brightness andefficiency.

In the view of Applicants, drawbacks or limitations of existingdirect-lit backlights include inefficiencies associated with thestrongly diffusing plate; in the case of LED sources, the need for largenumbers of such sources for adequate uniformity and brightness, withassociated high component cost and heat generation; and limitations onachievable thinness of the backlight beyond which light sources producenon-uniform and undesirable “punchthrough,” wherein a bright spotappears in the output area above each source.

In some cases, a direct-lit backlight may also include one or some lightsources at the periphery of the backlight, or an edge-lit backlight mayinclude one or some light sources directly behind the output area. Insuch cases, the backlight is considered “direct-lit” if most of thelight originates from directly behind the output area of the backlight,and “edge-lit” if most of the light originates from the periphery of theoutput area of the backlight.

Backlights of one type or another are usually used with liquid crystal(LC)-based displays. Liquid crystal display (LCD) panels, because oftheir method of operation, utilize only one polarization state of light,and hence for LCD applications it may be important to know thebacklight's brightness and uniformity for light of the correct oruseable polarization state, rather than simply the brightness anduniformity of light that may be unpolarized. In that regard, with allother factors being equal, a backlight that emits light predominantly orexclusively in the useable polarization state is more efficient in anLCD application than a backlight that emits unpolarized light.Nevertheless, backlights that emit light that is not exclusively in theuseable polarization state, even to the extent of emitting randomlypolarized light, are still fully useable in LCD applications, since thenon-useable polarization state can be easily eliminated by an absorbingpolarizer provided at the back of the LCD panel.

BRIEF SUMMARY

The present application discloses, inter alia, reflective and/ortransmissive films, surfaces, or other components that have a definedcombination of diffuse and specular characteristics. These componentsare referred to herein as “semi-specular,” and can be characterized by aquantity known as “transport ratio,” which is a function of incidenceangle. When properly placed within a suitable hollow recycling cavitybacklight with an output surface (front surface) that has a high valueof hemispheric reflectivity (R^(f) _(hemi)), they can help improve theoutput properties of the backlight and allow for the construction ofbacklights in new design spaces.

The application discloses, for example, backlights that include a frontand back reflector forming a hollow light recycling cavity, the frontreflector being partially transmissive to provide an output illuminationarea, and one or more light sources disposed to emit light into thelight recycling cavity over a limited angular distribution.Significantly, the backlights also include a component that provides thecavity with a desired balance of specular and diffuse characteristics,the component being characterized by a transport ratio greater than 15%at a 15 degree incidence angle and less than 95% at a 45 degreeincidence angle. The front or back reflectors can be or include thecomponent, or the component can be distinct from the front and backreflectors. In some cases, the transport ratio of the component isgreater than 20% at a 15 degree incidence angle, or less than 90% at a45 degree incidence angle.

The light sources, which may include a small area LED source and awedge-shaped reflector, may emit light into the light recycling cavitywith a restricted or partially collimated angular distribution. Forexample, the injected light may be collimated to have a full angle-widthat half maximum power (FWHM) centered about a transverse plane parallelto the backlight output area in a range from 0 to 60 degrees, or 0 to 30degrees. In such cases, the front reflector of the recycling cavitydesirably has a reflectivity that generally increases with angle ofincidence, and a transmission that generally decreases with angle ofincidence. Such reflectivity and transmission may be for unpolarizedvisible light in any plane of incidence, or for light of a useablepolarization state incident in a plane for which oblique light of theuseable polarization state is p-polarized.

In another aspect, the present disclosure provides a hollow lightrecycling cavity including a front and back reflector, the frontreflector being partially transmissive to provide an output illuminationarea, where the cavity includes a cavity transport value of greater thanabout 0.5 and less than about 0.95, and further where the frontreflector includes an R_(hemi) of greater than about 0.6.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,where like reference numerals designate like elements, and wherein:

FIG. 1 is a schematic side view of a generalized recycling backlight orsimilar extended area source;

FIG. 1 a is a perspective view of a surface, showing different planes ofincidence and different polarization states;

FIG. 2 is a schematic side view of an edge-lit backlight containing asolid light guide;

FIG. 3 is a schematic side view of an edge-lit backlight containing ahollow recycling cavity;

FIG. 4 is a schematic side view of an edge-lit backlight containing ahollow recycling cavity and a light source member disposed to emit lightinto the cavity over a limited angular distribution;

FIG. 5 is a schematic side view of an edge-lit backlight containing asolid light guide, demonstrating light injection principles;

FIG. 6 is a schematic top view of an edge-lit backlight containing asolid light guide, also demonstrating light injection principles;

FIG. 7 is a schematic side view of a direct-lit backlight containing ahollow recycling cavity;

FIGS. 8-10 are schematic side views of backlights containing a hollowrecycling cavity, comparing the effects of specular, Lambertian, andsemi-specular reflectors;

FIG. 11 is a schematic side view of a direct-lit backlight containing ahollow recycling cavity;

FIG. 12 is a conoscopic plot of reflected light for a sample film;

FIG. 13 a is a graph of measured luminance vs. observation angle for thefilm of FIG. 12, the luminance measured in the plane of incidence(Phi=0);

FIG. 13 b is a graph of measured luminance vs. observation angle for thefilm of FIG. 12, the luminance measured in a plane perpendicular to theplane of incidence (Phi=90);

FIG. 14 is a plot of transport ratio T vs. angle of incidence for avariety of films sampled;

FIGS. 15 a-b, 16, 17 a-b, 19 a-b, and 20 a-b are plots of measuredluminance vs. observation angle for other tested films; and

FIGS. 18 a-b are a conoscopic plots of reflected light for the film ofExamples G-H, where the film has a first orientation relative to theplane of incidence for Example G, and a second orientation relative tothe plane of incidence for Example H, and the first orientation isrotated 90 degrees relative to the second orientation.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

It would be beneficial for next generation backlights to combine some orall of the following characteristics while providing a brightness andspatial uniformity that is adequate for the intended application: thinprofile; design simplicity, such as a minimal number of film componentsand a minimal number of sources, and convenient source layout; lowweight; no use of or need for film components having substantial spatialnon-uniformities from one position in the backlight to another (e.g., nosignificant gradation); compatibility with LED sources, as well as othersmall area, high brightness sources such as solid state laser sources;insensitivity to problems associated with color variability among LEDsources that are all nominally the same color; to the extent possible,insensitivity to the burnout or other failure of a subset of LEDsources; and the elimination or reduction of at least some of thelimitations and drawbacks mentioned in the Background section above.

Whether these characteristics can be successfully incorporated into abacklight depends in part on the type of light source used forilluminating the backlight. CCFLs, for example, provide white lightemission over their long narrow emissive areas, and those emissive areascan also operate to scatter some light impinging on the CCFL, such aswould occur in a recycling cavity. The typical emission from a CCFL,however, has an angular distribution that is substantially Lambertian,and this may be inefficient or otherwise undesirable in a givenbacklight design. Also, the emissive surface of a CCFL, althoughsomewhat diffusely reflective, also typically has an absorptive lossthat Applicants have found to be significant if a highly recyclingcavity is desired. An LED die also emits light in a Lambertian manner,but because of its much smaller size relative to CCFLs, the LED lightdistribution can be readily modified, e.g., with an integral encapsulantlens reflector, or extractor to make the resulting packaged LED aforward-emitter, a side-emitter, or other non-Lambertian profile. Suchnon-Lambertian profiles can provide important advantages for thedisclosed backlights. However, the smaller size and higher intensity ofLED sources relative to CCFLs can also make it more difficult to producea spatially uniform backlight output area using LEDs. This isparticularly true in cases where individual colored LEDs, such asarrangements of red/green/blue (RGB) LEDs, are used to produce whitelight, since failure to provide adequate lateral transport or mixing ofsuch light can easily result in undesirable colored bands or areas.White light emitting LEDs, in which a phosphor is excited by a blue orUV-emitting LED die to produce intense white light from a small area orvolume on the order of an LED die, can be used to reduce such colornon-uniformity, but white LEDs currently are unable to provide LCD colorgamuts as wide as those achievable with individual colored LEDarrangements, and thus may not be desirable for all end-useapplications.

Applicants have discovered combinations of backlight design featuresthat are compatible with LED source illumination, and that can producebacklight designs that outperform backlights found in state-of-the-artcommercially available LCD devices in at least some respects. Thesebacklight design features include some or all of the following:

-   -   a recycling optical cavity in which a large proportion of the        light undergoes multiple reflections between substantially        coextensive front and back reflectors before emerging from the        front reflector, which is partially transmissive and partially        reflective;    -   overall losses for light propagating in the recycling cavity are        kept extraordinarily low, for example, both by providing a        substantially enclosed cavity of low absorptive loss, including        low loss front and back reflectors as well as side reflectors,        and by keeping losses associated with the light sources very        low, for example, by ensuring the cumulative emitting area of        all the light sources is a small fraction of the backlight        output area;    -   a recycling optical cavity that is hollow, i.e., the lateral        transport of light within the cavity occurs predominantly in        air, vacuum, or the like rather than in an optically dense        medium such as acrylic or glass;    -   in the case of a backlight designed to emit only light in a        particular (useable) polarization state, the front reflector has        a high enough reflectivity for such useable light to support        lateral transport or spreading, and for light ray angle        randomization to achieve acceptable spatial uniformity of the        backlight output, but a high enough transmission into the        appropriate application-useable angles to ensure application        brightness of the backlight is acceptable;    -   the recycling optical cavity contains a component or components        that provide the cavity with a balance of specular and diffuse        characteristics, the component having sufficient specularity to        support significant lateral light transport or mixing within the        cavity, but also having sufficient diffusivity to substantially        homogenize the angular distribution of steady state light within        the cavity, even when injecting light into the cavity only over        a narrow range of angles (and further, in the case of a        backlight designed to emit only light in a particular (useable)        polarization state, recycling within the cavity preferably        includes a degree of randomization of reflected light        polarization relative to the incident light polarization state,        which allows a mechanism by which non-useable polarized light is        converted into useable polarized light);    -   the front reflector of the recycling cavity has a reflectivity        that generally increases with angle of incidence, and a        transmission that generally decreases with angle of incidence,        where the reflectivity and transmission are for unpolarized        visible light and for any plane of incidence, and/or for light        of a useable polarization state incident in a plane for which        oblique light of the useable polarization state is p-polarized        (and further, the front reflector has a high value of        hemispheric reflectivity and while also having a sufficiently        high transmission of application-useable light);    -   light injection optics that partially collimate or confine light        initially injected into the recycling cavity to propagation        directions close to a transverse plane (the transverse plane        being parallel to the output area of the backlight), e.g., an        injection beam having a full angle-width (about the transverse        plane) at half maximum power (FWHM) in a range from 0 to 90        degrees, or 0 to 60 degrees, or 0 to 30 degrees. In some        instances it may be desirable for the maximum power of the        injection light to have a downward projection, below the        transverse plane, at an angle with the transverse plane of no        greater than 40 degrees, and in other instances, to have the        maximum power of the injected light to have an upwards        projection, above the transverse plane towards the front        reflector, at an angle with the transverse plane of no greater        than 40 degrees.

Backlights for LCD panels, in their simplest form, consist of lightgeneration surfaces, such as the active emitting surfaces of LED dies orthe outer layers of phosphor in a CCFL bulb, and a geometric and opticalarrangement of distributing or spreading this light in such a way as toproduce an extended- or large-area illumination surface or region,referred to as the backlight output area, which is spatially uniform inits emitted brightness. Generally, this process of transforming veryhigh brightness local sources of light into a large-area uniform outputsurface results in a loss of light because of interactions with all ofthe backlight cavity surfaces, and interaction with the light-generationsurfaces. To a first approximation, any light that is not delivered bythis process through the output area or surface associated with a frontreflector—optionally into a desired application viewer-cone (if any),and with a particular (e.g., LCD-useable) polarization state (if any)—is“lost” light. In a commonly assigned related application, we describe amethodology of uniquely characterizing any backlight containing arecycling cavity by two essential parameters. This related PCT PatentApplication is entitled “Thin Hollow Backlights With Beneficial DesignCharacteristics” (Attorney Docket No. 63031WO003).

We now turn our attention to a generalized backlight 10 shown in FIG. 1,in which a front reflector 12 and a back reflector 14 form a hollowlight recycling cavity 16. The backlight 10 emits light over an extendedoutput area or surface 18, which in this case corresponds to an outermajor surface of the front reflector 12. The front and back reflectorsare shown plane and parallel to each other, and coextensive over atransverse dimension 13, which dimension also corresponds to atransverse dimension such as a length or width of the output area 18.The front reflector reflects a substantial amount of light incident uponit from within the cavity, as shown by an initial light beam 20 beingreflected into a relatively strong reflected beam 20 a and a relativelyweaker transmitted beam 20 b. Note that the arrows representing thevarious beams are schematic in nature, e.g., the illustrated propagationdirections and angular distributions of the different beams are notintended to be completely accurate.

Returning to the figure, reflected beam 20 a is strongly reflected byback reflector 14 into a beam 20 c. Beam 20 c is partially transmittedby front reflector 12 to produce transmitted beam 20 d, and partiallyreflected to produce another beam (not shown). The multiple reflectionsbetween the front and back reflectors help to support transversepropagation of light within the cavity, indicated by arrow 22. Thetotality of all transmitted beams 20 b, 20 d, and so on add togetherincoherently to provide the backlight output.

For illustrative purposes, small area light sources 24 a, 24 b, 24 c areshown in alternative positions in the figure, where source 24 a is shownin an edge-lit position and is provided with a reflective structure 26that can help to collimate (at least partially) light from the source 24a. Sources 24 b and 24 c are shown in direct-lit positions, and source24 c would generally be aligned with a hole or aperture (not shown)provided in the back reflector 14 to permit light injection into thecavity 16. Reflective side surfaces (not shown, other than reflectivestructure 26) would typically also be provided generally at theendpoints of dimension 13, preferably connecting the front and backreflectors 12, 14 in a sealed fashion for minimum losses. In somedirect-lit embodiments, generally vertical reflective side surfaces mayactually be thin partitions that separate the backlight from similar oridentical neighboring backlights, where each such backlight is actuallya portion of a larger zoned backlight. Light sources in the individualsub-backlights can be turned on or off in any desired combination toprovide patterns of illuminated and darkened zones for the largerbacklight. Such zoned backlighting can be used dynamically to improvecontrast and save energy in some LCD applications.

A backlight cavity, or more generally any lighting cavity, that convertsline or point sources of light into uniform extended area sources oflight can be made using a combination of reflective and transmissiveoptical components. In many cases, the desired cavity is very thincompared to its lateral dimension. Preferred cavities for providinguniform extended area light sources are those that create multiplereflections that both spread the light laterally and randomize the lightray directions. Generally, the smaller the area of the light sourcescompared to the area of the front face, the greater the problem increating a uniform light intensity over the output region of the cavity.

Historically, solid light guides have generally been used for thethinnest backlights and, except for very small displays such as thoseused in handheld devices, have been illuminated with linearly continuouslight sources such as cold cathode fluorescent lights (CCFLs). A solidlight guide provides low loss transport of light and specularreflections at the top and bottom surfaces of the light guide via thephenomenon of total internal reflection (TIR) of light. As describedelsewhere in this application, the specular reflection of light providesthe most efficient lateral transport of light within a light guide.Extractors placed on the top or bottom surface of a solid light guideredirect the light in order to direct it out of the light guide,creating in essence, a partial reflector.

Solid light guides, however, present several problems for largedisplays, such as cost, weight, and light uniformity. The problem withuniformity for large area displays has increased with the advent ofseparate red/green/blue (RGB) colored LEDs, which are effectively pointsources of light compared to the much larger area of the output regionof the backlight. The high intensity point sources can cause uniformityproblems with conventional direct-lit backlights as well as edge-litsystems that utilize solid light guides. The uniformity problems can begreatly reduced if a hollow light guide could be made that also providesfor significant lateral transport of light as in a solid light guide. Insome cases for polarization and light ray angle recycling systems, ahollow cavity can be more proficient at spreading light laterally acrossa display face than a solid cavity. Some of the components that can beused to accomplish this effectively for a hollow light guide have notgenerally been available to the backlight industry, or in cases wherethe components already existed, the hollow light guides have not untilnow been constructed in the correct fashion to make a uniform, thin,efficient hollow light mixing cavity.

An efficient hollow reflective cavity has several advantages over asolid light guide for making a thin uniform backlight, even though asolid light guide does provide efficient top and bottom reflectors viathe phenomenon of Total Internal Reflection (TIR). The solid light guideis used primarily to provide a lateral dispersion of the light beforethe light interacts with other components such as reflective polarizersand other brightness enhancement films.

The TIR surfaces of a solid guide, however, are inadequate to meet allthe needs of modern backlights, and additional light control films aretypically added both above and below the solid light guide. Most systemsthat use a solid light guide today also use a separate back reflector toutilize brightness enhancement films such as BEF and DBEF. These filmsrecycle light that is extracted from the light guide but is un-useablefor the display because of unsuitable polarization or angle ofpropagation. The back reflector is typically a white reflector, which issubstantially Lambertian in its reflection characteristics. Much of thelateral transport, however, is first achieved with the TIR surfaces ofthe solid guide, and the recycled light is converted and returned to thedisplay with the Lambertian back reflector. If separate top and bottomlight management films are required anyway, it can be more efficient touse them alone to create a hollow light guide and also to simultaneouslyprovide the functions of a reflective polarizer and other brightnessenhancement films. In this manner, the solid guide, as well as otherbrightness enhancement films, can be omitted.

We propose replacing the solid light guide with air, and the TIRsurfaces of a solid light guide with high efficiency low-loss specularand semi-specular reflectors. As explained below, these types ofreflectors can be important for facilitating optimal lateral transportof the light within the backlight cavity. Lateral transport of light canbe initiated by the optical configuration of the light source, or it canbe induced by an extensive recycling of light rays in a cavity thatutilizes low loss semi-specular reflectors.

We can replace the TIR surfaces of the solid light guide with spatiallyseparated low loss reflectors that fall into two general categories. Oneis a partial reflector for the front face and the second is a fullreflector for the back and side faces. As described above, the latterare often added to solid light guide systems anyway. For optimaltransport of light and mixing of light in the cavity, both the front andback reflectors may be specular or semi-specular instead of Lambertian.Further, a semi-specular component of some type may be useful somewherewithin the cavity to promote uniform mixing of the light. The use of airas the main medium for lateral transport of light in large light guidesenables the design of lighter, lower cost, and more uniform displaybacklights.

For a hollow light guide to significantly promote the lateral spreadingof light, the means of light injection into the cavity is important,just as it is in solid light guides. The format of a hollow light guideallows for more options for injecting light at various points in adirect lit backlight, especially in backlights with multiple butoptically isolated zones. In a hollow light guide system, the functionof the TIR and Lambertian reflectors can be accomplished with thecombination of a specular reflector and a semi-specular, forwardscattering diffusion element. As explained below, excessive use ofLambertian scattering or reflecting elements is not considered optimal.

Exemplary partial reflectors (front reflectors) we describehere—particularly, for example, the asymmetric reflective films (ARFs)described in commonly assigned PCT Patent Application entitled BACKLIGHTAND DISPLAY SYSTEM USING SAME (Attorney Docket No. 63274WO004)—providefor low loss reflections and also for better control of transmission andreflection of polarized light than is possible with TIR in a solid lightguide alone. Thus, in addition to improved light distribution in alateral sense across the face of the display, the hollow light guide canalso provide for improved polarization control for large systems.Significant control of transmission with angle of incidence is alsopossible with the preferred ARFs mentioned above. In this manner, lightfrom the mixing cavity can be collimated to a significant degree as wellas providing for a polarized light output with a single filmconstruction.

Preferred front reflectors have a relatively high overall reflectivityto support relatively high recycling within the cavity. We characterizethis in terms of “hemispheric reflectivity,” meaning the totalreflectivity of a component (whether a surface, film, or collection offilms) when light (of a wavelength range of interest) is incident on itfrom all possible directions. Thus, the component is illuminated withlight incident from all directions (and all polarization states, unlessotherwise specified) within a hemisphere centered about a normaldirection, and all light reflected into that same hemisphere iscollected. The ratio of the total flux of the reflected light to thetotal flux of the incident light for the wavelength range of interestyields the hemispheric reflectivity, R_(hemi). Characterizing areflector in terms of its R_(hemi) is especially convenient forrecycling cavities because light is generally incident on the internalsurfaces of the cavity—whether the front reflector, back reflector, orside reflectors—at all angles. Further, unlike the reflectivity fornormal incidence, R_(hemi) is insensitive to, and already takes intoaccount, the variability of reflectivity with incidence angle, which maybe very significant for some components (e.g., prismatic films).

In fact, some embodiments of front reflectors exhibit a(direction-specific) reflectivity that increases with incidence angleaway from the normal (and a transmission that generally decreases withangle of incidence), at least for light incident in one plane.

Such reflective properties cause the light to be preferentiallytransmitted out of the front reflector at angles closer to the normal,i.e., closer to the viewing axis of the backlight, and this helps toincrease the perceived brightness of the display at viewing angles thatare important in the display industry (at the expense of lower perceivedbrightness at higher viewing angles, which are usually less important).We say that the increasing reflectivity with angle behavior is “at leastfor light incident in one plane,” because sometimes a narrow viewingangle is desired for only one viewing plane, and a wider viewing angleis desired in the orthogonal plane. An example is some LCD TVapplications, where a wide viewing angle is desired for viewing in thehorizontal plane, but a narrower viewing angle is specified for thevertical plane. In other cases, narrow angle viewing is desirable inboth orthogonal planes so as to maximize on-axis brightness.

When we discuss oblique angle reflectivity, it is helpful to keep inmind the geometrical considerations of FIG. 1 a. There, we see a surface50 that lies in an x-y plane, with a z-axis normal direction. If thesurface is a polarizing film or partially polarizing film (such as theARFs described in Attorney Docket No. 63274WO004), we designate forpurposes of this application the y-axis as the “pass axis” and thex-axis as the “block axis.” In other words, if the film is a polarizingfilm, normally incident light whose polarization axis is parallel to they-axis is preferentially transmitted compared to normally incident lightwhose polarization axis is parallel to the x-axis. Of course, ingeneral, the surface 50 need not be a polarizing film.

Light can be incident on surface 50 from any direction, but weconcentrate on a first plane of incidence 52, parallel to the x-z plane,and a second plane of incidence 54, parallel to the y-z plane. “Plane ofincidence” of course refers to a plane containing the surface normal anda particular direction of light propagation. We show in the figure oneoblique light ray 53 incident in the plane 52, and another oblique lightray 55 incident in the plane 54. Assuming the light rays to beunpolarized, they will each have a polarization component that lies intheir respective planes of incidence (referred to as “p-polarized” lightand labeled “p” in the figure), and an orthogonal polarization componentthat is oriented perpendicular to the respective plane of incidence(referred to as “s-polarized light” and labeled “s” in the figure). Itis important to note that for polarizing surfaces, “s” and “p” can bealigned with either the pass axis or the block axis, depending on thedirection of the light ray. In the figure, the s-polarization componentof ray 53, and the p-polarization component of ray 55, are aligned withthe pass axis (the y-axis) and thus would be preferentially transmitted,while the opposite polarization components (p-polarization of ray 53,and s-polarization of ray 55) are aligned with the block axis.

With this in mind, let us consider the meaning of specifying (if wedesire) that the front reflector “exhibit a reflectivity that generallyincreases with angle of incidence,” in the case where the frontreflector is an ARF such as is described in the Attorney Docket No.63274WO004 application referenced elsewhere. The ARF includes amultilayer construction (e.g., coextruded polymer microlayers that havebeen oriented under suitable conditions to produce desired refractiveindex relationships, and desired reflectivity characteristics) having avery high reflectivity for normally incident light in the blockpolarization state and a lower but still substantial reflectivity (e.g.,25 to 90%) for normally incident light in the pass polarization state.The very high reflectivity of block-state light (p-polarized componentof ray 53, and s-polarized component of ray 55) generally remains veryhigh for all incidence angles. The more interesting behavior is for thepass-state light (s-polarized component of ray 53, and p-polarizedcomponent of ray 55), since that exhibits an intermediate reflectivityat normal incidence. Oblique pass-state light in the plane of incidence52 will exhibit an increasing reflectivity with increasing incidenceangle, due to the nature of s-polarized light reflectivity (the relativeamount of increase, however, will depend on the initial value ofpass-state reflectivity at normal incidence). Thus, light emitted fromthe ARF film in a viewing plane parallel to plane 52 will be partiallycollimated or confined in angle. Oblique pass-state light in the otherplane of incidence 54 (i.e., the p-polarized component of ray 55),however, can exhibit any of three behaviors depending on the magnitudeand polarity of the z-axis refractive index difference betweenmicrolayers relative to the in-plane refractive index differences, asdiscussed in the 63274WO004 application.

In one case, a Brewster angle exists, and the reflectivity of this lightdecreases with increasing incidence angle. This produces bright off-axislobes in a viewing plane parallel to plane 54, which are usuallyundesirable in LCD viewing applications (although in other applicationsthis behavior may be acceptable, and even in the case of LCD viewingapplications this lobed output may be re-directed towards the viewingaxis with the use of a prismatic turning film).

In another case, a Brewster angle does not exist or is very large, andthe reflectivity of the p-polarized light is relatively constant withincreasing incidence angle. This produces a relatively wide viewingangle in the referenced viewing plane.

In the third case, no Brewster angle exists, and the reflectivity of thep-polarized light increases significantly with incidence angle. This canproduce a relatively narrow viewing angle in the referenced viewingplane, where the degree of collimation is tailored at least in part bycontrolling the magnitude of the z-axis refractive index differencebetween microlayers in the ARF.

Of course, the reflective surface 50 need not have asymmetric on-axispolarizing properties as with ARF. Symmetric multilayer reflectors, forexample, can be designed to have a high reflectivity but withsubstantial transmission by appropriate choice of the number ofmicrolayers, layer thickness profile, refractive indices, and so forth.In such a case, the s-polarized components of both ray 53 and 55 willincrease with incidence angle in the same manner with each other. Again,this is due to the nature of s-polarized light reflectivity, but therelative amount of increase will depend on the initial value of thenormal incidence reflectivity. The p-polarized components of both ray 53and ray 55 will have the same angular behavior as each other, but thisbehavior can be controlled to be any of the three cases mentioned aboveby controlling the magnitude and polarity of the z-axis refractive indexdifference between microlayers relative to the in-plane refractive indexdifferences, as discussed in the 63274WO004 application.

Thus, we see that the increase in reflectivity with incidence angle (ifpresent) in the front reflector can refer to light of a useablepolarization state incident in a plane for which oblique light of theuseable polarization state is p-polarized. Alternately, such increase inreflectivity can refer to the average reflectivity of unpolarized light,in any plane of incidence.

Preferred back reflectors also have a high hemispherical reflectivityfor visible light, typically, much higher than the front reflector sincethe front reflector is deliberately designed to be partiallytransmissive to provide the required light output of the backlight. Thehemispherical reflectivity of the back reflector is referred to as R^(b)_(hemi), while that of the front reflector is referred to as R^(f)_(hemi). It may be preferred that the product R^(f) _(hemi)*R^(b)_(hemi) is at least 70% (0.70), or 75%, or 80%.

There are several key aspects to the design of a hollow light recyclingcavity that are relevant to spreading light efficiently and uniformlyfrom small area sources to the full area of the output region. Theseare 1) proper directional injection of light into the cavity from thelight source; 2) the use of forward scattering diffusers orsemi-specular reflecting surfaces or components within the cavity; 3) afront reflector that transmits the light, but which is alsosubstantially reflective such that most light rays are recycled manytimes between the front and back reflector so as to eventually randomizethe light ray directions within the cavity; and 4) minimizing losses byoptimal component design.

Conventional backlights have used one or more of these techniques toenhance the uniformity of the backlight, but never all foursimultaneously in the correct configuration for a thin and hollowbacklight having very small area light sources. These aspects of cavitydesign are examined in more detail herein.

With regard to the injection of light from the light source into thecavity, hollow light guides have a significantly different requirementfor light injection compared to solid light guides. For example, theedge-lit solid guide as shown in FIG. 10 of U.S. Pat. No. 6,905,220(Wortman et. al.) illustrates edge injection into a solid guide simplyby placing a fluorescent tube against one edge of the solid light guide.An analogous arrangement of a backlight 210 is shown in FIG. 2 of thepresent application. The fluorescent tube 224 is a Lambertian emitter,i.e., light emanates equally in all directions. Wrapping ¾ of the tubewith a mirror 226 again produces a Lambertian light field incident onthe flat vertical edge 242 of the solid light guide 240. A simpleapplication of Snell's law shows that even the highest angle rays (+/−90deg) incident on that surface (and transmitted to the interior of thesolid guide) will all be totally internally reflected upon their firstencounter with a front or back surface 244, 246, unless they encounteran extraction feature. In this manner, light rays are transportedefficiently across the solid light guide 240.

If the solid light guide is replaced with a hollow light guide, none ofthe light rays are refracted upon entering the hollow guide. With aLambertian distribution of light rays entering from one edge, there willbe a large amount of light directed in the vertical direction (towardsthe front reflector), as illustrated in FIG. 3, where backlight 310includes a hollow light guide 316 and a fluorescent tube 324. To make auniform backlight, the partially reflective film should be extremelyreflective near the source and then have a highly graded transmissivityacross the face of the cavity—more than what is needed with the gradedextraction pattern of a solid guide.

A more uniform hollow backlight can be made by using a partiallycollimated light source, or a Lambertian source with collimating opticalmeans, to produce a highly directional source that promotes the lateraltransport of light. An example of such a light injector 426 is shown inFIG. 4, where backlight 410 includes a hollow light guide 416, one ormore light sources 424, and injector 426. Examples of other suitablelight injectors are described in commonly assigned PCT PatentApplication entitled COLLIMATING LIGHT INJECTORS FOR EDGE-LIT BACKLIGHTS(Attorney Docket No. 63034WO004). Any suitable technique can be utilizedto provide the desired degree of collimation and angle of injection oflight from the light sources, e.g., compound parabolicconcentrator-shaped light injectors, lenses, extractors, etc.

In some embodiments, the light rays are preferably injected into ahollow light guide with a predominantly horizontal direction, i.e.,having a collimation characteristic that provides a small angle-width athalf maximum (FWHM), with the light collimated with a degree of symmetryabout the transverse plane. Some finite distribution of ray anglescannot be avoided, and this distribution can be optimized by the shapeof the collimating optics in conjunction with the emission pattern ofthe light source to help provide uniformity of the light across theoutput area of the cavity. The partially reflecting front reflector andthe partial diffusion of the semi-specular reflector produce a lightrecycling and randomizing light cavity that works in harmony with theinjection light sources optics to create a uniform, thin, and efficienthollow light guide.

In some embodiments, brightness uniformity of an edge-lit backlight canbe enhanced by aiming the injection light output direction, adjustingspacing between adjacent light sources or groups of light sources, or acombination of the two techniques. For example, forward emitting lightsources having narrow light distribution cone angles as described hereincan be selected as a way of controlling the direction of light emittedby the light sources. Typically, for edge-lit backlights, the lightsources can be arranged along one or more edges of a backlight such thatthe emitted beams are directed substantially perpendicular to the inputedge or edges and parallel to each other. By aiming the beams of one ormore light sources in a non-perpendicular direction and towards selectedareas of the backlight, the brightness of the selected area can beincreased with a corresponding brightness decrease in other areas.

For example, in a backlight having several LEDs uniformly disposed alongone edge, the LEDs can be aimed such that all of the beams intersect atthe approximate center of the backlight, thus resulting in a brightcenter and less bright edges. If fewer than all of the beams aredirected to intersect at the center, the center brightness can bedecreased, thereby providing a mechanism to adjust the brightness to adesired level. Analogous arrangements can be used to produce, forexample, brighter edges and a less bright center. Any suitable techniquecan be used to control the emission direction of the light sources,e.g., mounting orientation of the light sources, lenses, extractors,collimating reflectors, etc. In general, the light sources can be aimedsuch that the light is predominantly directed at any suitable anglerelative to the transverse plane, including 0 degrees.

Light sources can also be arranged along one or more edges of abacklight such that spacing between them is non-uniform. In thissituation, the part of the backlight having more closely spaced lightsources will be brighter. For example, in a backlight having 40 LEDsdisposed along one edge, the center 20 LEDs can be more closely spacedthan the flanking 10 LEDs towards each edge, thus producing a brightercenter. Analogous adjustments can be used to produce brighter edges.

In some embodiments, one or more optical elements can be positionedbetween the light sources and the entrance to the cavity. Any suitableoptical element can be included. For example, one or more absorptive orreflective filters can be positioned between the light sources and thecavity to inject a desired light flux distribution into the cavity.Other types of filters can be provided to reduce or remove UV or shortwavelength light from the injected light to reduce photodegradation inthe backlight cavity materials. Other suitable film or films includemultilayer optical films (e.g., DBEF, APF, asymmetric reflective films),light redirecting films (e.g., BEF), etc.

Further, for example, the optical element can include a film or layerhaving a phosphor coating to convert light from one or more lightsources having one particular optical characteristic (e.g., wavelength)into a second optical characteristic. See, e.g., U.S. Pat. No. 7,255,469(Wheatley et al.), entitled PHOSPHOR BASED ILLUMINATION SYSTEM HAVING ALIGHT GUIDE AND AN INTERFERENCE REFLECTOR.

The one or more optical elements can also include any suitable structureor structures to modify the direction of the light that is injected intothe cavity, e.g., refractive structures, reflective structures, anddiffractive structures.

We now discuss some advantages of hollow cavities over solid lightguides. Even though light can move large distances laterally in a lowloss solid light guide, light that exits the light guide and is recycledfor polarization or angle conversion cannot thereafter participate insubstantial lateral spreading of light. The reason is best explainedwith the ray diagram shown in FIG. 5, which depicts a schematic sideview of a backlight 510 containing a solid light guide 540. Light ray550 is refracted towards the normal as it re-enters the light guide 540after reflection by a partial reflecting film 548. If the light guide540 were hollow, the ray would proceed on path 552 that would offer amuch greater lateral propagation of the light ray. This effect issubstantial for light rays entering the light guide 540 from air atangles of about 30 degrees or more. If the light ray were to re-enterthe solid light guide 540 at another extraction dot, which scatters thelight over many angles, some of the light would be spread laterally athigh angles, but some would re-enter at very small angles. The neteffect is a decreased lateral propagation of the light.

Referring still to FIG. 5, the refraction of the initial light ray 554towards the lateral direction upon transmission as light ray 556 wouldappear to offer a significant advantage to a solid light guide systemfor spreading light laterally. However, this is true only for onecross-sectional plane or perspective. By viewing the system from above(i.e., from the front of the backlight), as shown in the plan view ofFIG. 6, one can see that a hollow light guide is superior for providinguniform light intensity along a direction orthogonal to the plane ofFIG. 5. Light ray 654 in FIG. 6 is refracted toward the local surfacenormal to produce light ray 656, causing it to spread very little in thedirection parallel to the left edge (from the perspective of FIG. 6) ofthe light guide 640. If the light guide 640 of FIG. 6 were hollow, thenlight ray 654 would not be refracted into the path of light ray 656.Instead, it would follow path 658, thereby better filling in the “gaps”between the point light sources at the edge of the light guide.

FIGS. 2-6 depict edge-lit systems, but the same principles apply todirect-lit systems. For direct-lit systems, the light sources are eitherinside the cavity, or a hole or portal is made in the back reflectorthrough which the light can enter the cavity from a source. In eithercase, an opening is made in the back reflector, preferably as small aspossible, so that the light source can be inserted or its light can passthrough the back reflector with minimal effect on the overall averagereflectivity of the back reflector.

In direct-lit systems, it is generally preferable that only smallamounts of the light from a given light source are directly incident onthe front reflector in regions of the output area directly opposing thatsource. One approach for achieving this is a packaged LED or the likedesigned to emit light mostly in the lateral directions. This feature istypically achieved by the optical design of the LED package,specifically, the encapsulant lens. Another approach is to place alocalized reflector above the LED to block its line of sight of thefront reflector. Any high efficiency mirror can be used for thispurpose. Preferably, the mirror is curved in a convex shape so as tospread the reflected light away from the source so it is not reabsorbed.This arrangement also imparts substantial lateral components to thelight ray direction vectors. A refractive element such as a lens or aFresnel lens having a negative focal length (i.e., a diverging lens) maybe used for this purpose as well. Still another approach is covering thelight source with a piece of a reflective polarizer that is misalignedwith respect to a polarization pass axis of the front reflector. Thelight transmitted by the local reflective polarizer proceeds to thefront reflector where it is mostly reflected and recycled, therebyinducing a substantial lateral spreading of the light. Reference is madein this regard to commonly assigned U.S. Patent Publication No.2006/0187650 (Epstein et al.), entitled DIRECT LIT BACKLIGHT WITH LIGHTRECYCLING AND SOURCE POLARIZERS.

There may be instances where Lambertian emitting LEDs are preferred in adirect-lit backlight for reasons of manufacturing cost or efficiency.Individual light deflection devices may not be preferred for similarreasons. Good uniformity may still be achieved with such a cavity byimposing a greater degree of recycling in the cavity. This may beachieved by using a front reflector that is even more highly reflective,e.g., having less than about 10% or 20% total transmission andconversely 90% or 80% reflection. For a polarized backlight, thisarrangement further calls for a block axis of the front reflector havinga very low transmission on the order of 1% to 2% or less. An extremeamount of recycling, however, may lead to unacceptable losses in thecavity.

If localized reflectors or deflectors are unacceptable as a solution tohiding Lambertian light sources, a third component 760 that iscoextensive with the front and back reflectors 712, 714 can be added tothe cavity 716, as shown in FIG. 7. This third component 760 can be adiffuser, such as a standard volume diffuser, a close-packed array ofdiverging Fresnel lenses, or another partial reflector. This thirdcomponent 760 need not be polarizing if the front reflector 712 ispolarizing. If lens arrays are used, the lenses can be linear, circular,elliptical, or any suitable shape. Linear lenses are useful forspreading light in a direction perpendicular to a row of LEDs. Whateveris used for the third component, it is preferably constructed of verylow loss materials since it is being used within a cavity between veryreflective surfaces, and the recycled light will pass through thiscomponent many times.

Having reviewed some of the benefits and design challenges of hollowcavities relative to solid light guides, we now turn to a detailedexplanation and exposition of semi-specular reflective and transmissivecomponents, and advantages of using them rather than solely Lambertianor specular components in hollow recycling cavity backlights.

A pure specular reflector, sometimes referred to as a mirror, performsaccording to the optical rule that “the angle of incidence equals theangle of reflection.” This is seen in the hollow cavity 816 of FIG. 8.There, the front and back reflectors, 812, 814 are both purely specular.A small portion of an initially launched oblique light ray 850 istransmitted through the front reflector 812, but the remainder isreflected at an equal angle to the back reflector 814, and reflectedagain at an equal angle to the front reflector 812, and so on asillustrated. This arrangement provides maximum lateral transport of thelight across the cavity 816, since the recycled ray is unimpeded in itslateral transit of the cavity 816. However, no angular mixing occurs inthe cavity, since there is no mechanism to convert light propagating ata given incidence angle to other incidence angles.

A purely Lambertian reflector, on the other hand, redirects light raysequally in all directions. This is seen in the hollow cavity 916 of FIG.9, where the front and back reflectors 912, 914 are both purelyLambertian. The same initially launched oblique light ray 950 isimmediately scattered in all directions by the front reflector 912, mostof the scattered light being reflected back into the cavity 916 but somebeing transmitted through the front reflector 912. Some of the reflectedlight travels “forward” (generally to the right as seen in the figure),but an equal amount travels “backward” (generally to the left). Byforward scattering, we refer to the lateral or in-plane (in a planeparallel to the scattering surface in question) propagation componentsof the reflected light. When repeated, this process greatly diminishesthe forward directed component of a light ray after several reflections.The beam is rapidly dispersed, producing minimal lateral transport.

A semi-specular reflector provides a balance of specular and diffusiveproperties. In the hollow cavity 1016 of FIG. 10, the front reflector1012 is purely specular but the back reflector 1014 is semi-specular.The reflected portion of the same initially launched oblique light ray1050 strikes the back reflector 1018, and is substantiallyforward-scattered in a controlled amount. The reflected cone of light isthen partially transmitted but mostly reflected (specularly) back to theback reflector 1014, all while still propagating to a great extent inthe “forward” direction.

Semi-specular reflectors can thus be seen to promote the lateralspreading of light across the recycling cavity, while still providingadequate mixing of light ray directions and polarization. Reflectorsthat are partially diffuse but that have a substantially forwarddirected component will transport more light across a greater distancewith fewer total reflections of the light rays. In a qualitative way, wecan describe a semi-specular reflector as one that providessubstantially more forward scattering than reverse scattering. Asemi-specular diffuser can be defined as one that does not reverse thenormal component of the ray direction for a substantial majority of theincident light, i.e., the light is substantially transmitted in theforward (z) direction and scattered to some degree in the x and ydirections. A more quantitative description of semi-specular is givenbelow in connection with the examples.

FIGS. 8-10 illustrate cavity designs that use only two major opticalcomponents: the front and back reflectors. In this case, at least one ofthe reflectors should be semi-specular. The other can be specular, orsemi-specular, or even Lambertian, although semi-specular isadvantageous for efficiency and uniformity. As an alternative to thetwo-component systems of FIG. 10, additional optical components can beinserted into the recycling cavity between the front and backreflectors, and such additional components may be tailored to providethe desired degree of semi-specularity to the cavity. A simple exampleis shown in FIG. 11. In this case a semi-specular diffusing film 1170 issuspended in the cavity 1116 between the front and back reflectors 1112,1114, both of which can be specular or semi-specular. Although it isoften desirable to minimize the number of components in the cavity, theuse of a third component can sometimes provide a higher efficiencycavity by allowing for the minimal loss design of the front or backreflector.

The mixing of light rays in the cavity with forward scattering elementscan be accomplished in several ways. It can be done by diffusingelements that are either an integral part of the front or back reflectoror are laminated to the front or back reflector, or by use of a separatediffusing sheet placed anywhere between the two, as shown in FIG. 11.Combinations of any of these options are also possible. The choicesdepend on the relative importance of matters such as optical losses,component cost, and convenience of manufacturing. The diffusing elementmay be attached to or an integral part of either the front or backreflector, or an air gap may be provided between the diffuser and thereflectors.

Whether the diffuser is an integral part of either reflector, orlaminated to either reflector, or placed in the cavity as a separatecomponent, the overall desired optical performance is one with anangular spreading function that is substantially narrower than aLambertian distribution for a ray that completes one round trip passagefrom the back reflector to the front and back again. A semi-specularreflector can have characteristics of both a specular and a Lambertianreflector or can be a well defined Gaussian cone about the speculardirection. The performance depends greatly on how it is constructed.Keeping in mind that the diffuser component can also be separate fromthe reflector, several possible constructions exist for the backreflector, such as (1) partial transmitting specular reflector plus ahigh reflectance diffuse reflector; (2) partial Lambertian diffusercovering a high reflectance specular reflector; (3) forward scatteringdiffuser plus a high reflectance specular reflector; or (4) corrugatedhigh reflectance specular reflector.

For each numbered construction, the first element listed is arranged tobe inside the cavity. The first element of constructions (1) through (3)can be continuous or discontinuous over the area of the back reflector.In addition, the first element could have a gradation of diffuserproperties, or could be printed or coated with additional diffuserpatterns that are graded. The graded diffuser is optional, but may bedesirable to optimize the efficiency of various backlight systems. Theterm “partial Lambertian” is defined to mean an element that onlyscatters some of the incident light. The fraction of light that isscattered by such an element is directed almost uniformly in alldirections. In construction (1), the partial specular reflector is adifferent component than that utilized for the front reflector. Thepartial reflector in this case can be either a spatially uniform film ofmoderate reflectivity, or it can be a spatially non-uniform reflectorsuch as a perforated multilayer or metallic reflector. The degree ofspecularity can be adjusted either by changing the size and number ofthe perforations, or by changing the base reflectivity of the film, orboth.

Construction (4) can be made by thermally embossing a multilayerpolymeric mirror film, or by physically corrugating such a film.Additionally, any surface with these shapes can be coated with ametallic or enhanced metal reflective film. Furthermore, thesemi-specular constructions of (1) through (3) can be corrugated orembossed in order to optimize their light transport properties.

The front reflector of the recycling cavity can be made semi-specularwith constructions that are similar to the back reflector, but with somenotable differences. Some constructions are (a) a partial reflectingLambertian diffuser plus a partial specular reflector; (b) a forwardscattering diffuser plus a partial specular reflector; (c) a forwardscattering partial reflector; or (d) various combinations of (a) through(c).

The elements of these constructions can be continuous or discontinuousover the area of the front reflector. For example, such elements couldhave a gradation of diffusing or reflecting properties, or both. Theycould be printed or coated with patterns that are graded. A gradeddiffuser is optional but may be desirable to optimize the efficiency ofvarious backlight systems. The term “partial Lambertian” refers to anelement that only scatters some of the incident light into a Lambertianpattern, the rest being scattered into some other angular distribution,such as specular.

Again, the first element listed is arranged to be inside the recyclingcavity. The first element of all three constructions can be continuousor discontinuous over the area of the partial reflector, and the firstelement can have a gradation of diffuser properties, or can be printedor coated with additional diffuser patterns that are graded.

One or both of the front and back reflectors can be specular if adiffuser is placed somewhere in the cavity. One of the reflectors canalso be Lambertian, but in general this is not an optimum construction,particularly for edge-lit backlights. In this case, the other reflectorshould be semi-specular or specular. The forward scattering diffuserscan be either a surface or a volume diffuser, and can be symmetric orasymmetric with respect to both direction or polarization state.

Quantitatively, the degree of semi-specularity (specular vs. Lambertiancharacteristic of a given reflector or other component) can beeffectively characterized by comparing the fluxes of the forward- andback-scattered light components, referred to as F and B respectively.The forward and back-scattered fluxes can be obtained from theintegrated reflection intensities (or integrated transmissionintensities in the case of optically transmissive components) over allsolid angles. The degree of semi-specularity can then be characterizedby a “transport ratio” T, given by

T=(F−B)/(F+B).

T ranges from 0 to 1 as one moves from purely specular to purelyLambertian. For a pure specular reflector there is no back-scatter(B=0), and therefore T=F/F=1. For a pure Lambertian reflector, theforward- and back-scattered fluxes are the same (F=B), and thus T=0.Examples with experimentally measured values are given below. Thetransport ratio for any real reflective or transmissive component is afunction of incidence angle. This is logical, because one would expectthe amount of forward-scattered light, for example, to be different fora near-normally incident ray than for a grazing-incident ray.

In connection with a recycling cavity, one can define an “effectivecavity transport ratio,” i.e., the transport ratio experienced by agiven incident ray after a complete circuit or cycle of the recyclingcavity. This quantity may be of interest, particularly in cavities thatcontain at least one semi-specular component and at least one additionalscattering component (whether semi-specular or Lambertian). Sincetransport ratio is in general a function of incidence angle, one couldevaluate or specify the effective cavity transport ratio in terms of theFWHM property of the collimated light injected into the cavity.

The transport ratio is well defined for a single interaction of a lightray at one angle with a reflector or a diffuser. A good recycling cavitycreates multiple interactions of a light ray at all angles with at leasttwo reflecting or diffusing components, and perhaps three or more suchcomponents. Since the transport ratio for a single interaction is afunction of the angle of incidence, the description of an overall cavitytransport ratio is therefore more complex than for a single component.An “effective cavity transport ratio” or more descriptively a “cavitytransport value,” is a measure of how well a cavity can spread injectedlight from the injection point to distant points in the cavity and stillrandomize it sufficiently to direct light uniformly towards a viewer. Asimple method of estimating relative cavity transport values is usefulfor judging the comparative merits of various combinations of specular,semi-specular, and Lambertian components. For this purpose, we definethe forward transport number fT for each component, which is expressedas

fT=F/(F+B),

where F and B are defined and measured as described herein but nowaveraged over all angles of a single interaction. Measurements atintervals of about 15 degrees or less from 15 to 75 degrees angle ofincidence are sufficient to give a proper average. F and B are therelative fractions of forward and backscattered light and by definition,F+B=1, giving simply fT=F which is the fraction of forward scatteredlight. The cavity transport value CT is then the product of the F valuesof the front and back reflector of the cavity:

CT=F _(front) *F _(back).

For example, a specular front reflector (F_(front)=1) and asemi-specular back reflector (F_(back)=0.75, and a transport ratio ofT=0.5) have an overall cavity transport value of CT=1*0.75=0.75.

As another example, if the front reflector is Lambertian so thatF_(front)=0.5 (T=0) and the back reflector is semi-specular so thatF_(back)=0.75 (T=0.5), then the overall cavity transport value isCT=0.5*0.75 =0.375. One would expect the latter cavity to transport muchless light to a given distance from the injection point than the firstexample cavity. This prediction is confirmed by experiment as describedherein.

For some applications, the front reflector may consist of a stack ofseveral components, such as a specular or a semispecular reflectorfollowed by a light redirecting layer or one or more diffusers that mayor may not be laminated to each other. The front and back reflectorseach can be defined as a collection of components assembled in aspecific order. The collective transport properties of all componentsthat make up the front reflector or the back reflector can be determinedwith one measurement. The effect of an individual component (e.g., afilm) on the transport properties of a stack of components depends onthe component's sequence and orientation in the stack and the propertiesof the other components in the stack. For at least these reasons, thestack can be measured as a whole. The components of the front reflectorcan be placed in the measuring device, such as the ones made byAutronics and by Radiant Imaging (Duvall, Wash., USA), with the insidecavity surface facing the measuring light beam.

The measurement of F and B described above for semi-specular reflectorsis done in reflection mode, which means that portions of the incidentbeam pass through the diffuse layer twice or reflect from it once. Ifthe diffuser were an intermediate component positioned somewhere in thecavity between the front and back reflectors, then light rays passthrough it twice in making one front to back cycle during the transportprocess. For this reason, we define the F and B values of anintermediate component as those measured in the same manner as adiffuser coated on a mirror. The intermediate component can be groupedwith either the front or the back reflector, and the combined transportproperties of the intermediate component and the chosen reflector can bemeasured together. If the majority of the light is injected into thecavity above an intermediate component (or though-holes in it frombelow), then the intermediate component can be grouped with the bottomreflector. If the majority of light is injected below an intermediatecomponent, then the intermediate component can be grouped with the frontreflector for transport measurement.

A cavity is defined as being semi-specular if the product CT is greaterthan about 0.5 and less than about 0.95 along at least one azimuthal(in-plane) direction within the cavity. In some embodiments, it may bepreferred that the semi-specular cavity have a CT that is greater thanabout 0.6. In other embodiments, it may be preferred that thesemi-specular cavity have a CT that is greater than about 0.7.

If an intermediate component is not entirely co-extensive with the frontand back reflectors, then the overall cavity transport CT can be takento be a weighted average of the CT values of the different areas of thecavity that contain the different components.

With most common diffusers, T ranges between 0 and 1 and F ranges from0.5 to 1.0. However, if a material that possesses some retroreflectiveproperties is used as a diffuser, T can be negative and can range from 0to −1 and F can range from 0 to 0.5. Examples of retroreflectivematerials include glass beads and prismatic structures with 90 degree,or near 90 degree angled facets. Solid prism arrays on a planarsubstrate (e.g., BEF) are retroreflective for light incident on theplanar side in the plane perpendicular to the linear direction of theprisms, but only over a limited range of angles such as less than +/−10degrees.

Hollow 90 degree faceted structures with specularly reflectiveindividual facets are retroreflective for the entire angle range of 0 to45 degrees and additionally have a forward transport of zero for allangles of incidence between 0 and 90 degrees for the directionperpendicular to the groove direction.

Asymmetric components such as BEF and its many variations, or asymmetricdiffusing materials such as blends or holographic structures, can createdifferent cavity transport values along various directions.

The values of CT obtained by multiplying the values of F are onlyrelative measures of the transport properties of a cavity. The numericalvalues of light intensity as a function of distance from the source alsodepend on cavity geometry and the reflectivity of the front and the backreflectors. The higher the reflectivity of the reflectors, the furtherthe light can be transported within the cavities. In exemplaryembodiments, R_(hemi) can be greater than 0.6 or even greater than 0.8for the front reflector, and greater than about 0.95 for the backreflector.

Cavity transport values were measured with an experimental cavity usingdifferent combinations of specular (e.g., ESR, average T≈1),semispecular-1 (e.g., BEF-III on ESR with an air gap, average of T≈0.67perpendicular to the prism length axis), semi-specular-2 (e.g., beadcoated ESR, average T≈0.4) and Lambertian (e.g., TIPS, average T≈0.02)reflectors as the front and back reflectors. Both the front and backreflectors were chosen in this experiment to have high reflectivity toeliminate the complexity of variable light loss along the length of thecavity. In this way, the light transport properties are the only majorvariable between samples.

Light was injected into one open end of a four inch wide cavity (½ inchhigh and 12 inches long) using a green laser that was aimed at 30degrees below the horizontal. A large area amorphous silicon solar cellcovered the other end of the cavity and served as a light detector whenconnected in series with an ammeter. The relative cavity transport, asmeasured by the amount of light collected by the detector at the end ofthe cavity, decreased in the following order as one would expect fromthe above analysis:(specular/specular)>(semi-specular/specular)>(Lambertian/specular)>(semi-specular/semi-specular)>(semi-specular/Lambertian)>(Lambertian/Lambertian).The second listed component was the bottom reflector, toward which thelaser light was first directed. The order of some of these combinationsmay change depending on the value of T for various other semi-specularcomponents. The relative intensities, normalized to the (ESR/ESR) case,which gave by far the highest transport values, are listed in thefollowing table. Note that the measured intensity is not linear with thecalculated values of CT. This is expected because the actual intensitydepends on many factors as described above. The CT values, however, givea good prediction of relative intensity in various cavity constructionsfor transporting light.

Measured Top/Bottom reflector Bottom light types Top Reflector F_(top)Reflector F_(Bottom) CT intensity Specular/specular* ESR 1 ESR 1 1.00 1Semi-specular-1/ ESR- 0.83 ESR 1 0.83 0.226 specular BEF-IIISemi-specular-2/ Beaded 0.7 ESR 1 0.70 0.163 specular ESR Semi-specular/ESR- 0.83 Beaded 0.7 0.58 0.058 semi-specular BEF-III ESR Lambertian/ 1TIPs 0.5 ESR 1 0.50 0.091 specular film Lambertian/ 1 TIPs 0.5 ESR- 0.830.42 0.047 semi-specular film BEF III Semi-specular-2/ Beaded 0.7 2 TIPs0.5 0.35 0.029 Lambertian ESR films Lambertian/ 1 TIPs 0.5 2 TIPs 0.50.25 0.024 Lambertian film films *To provide some initial forward andlateral scattering of light in this all specular case, a 1.7 cm × 6 cmpiece of BEF-III was placed grooves up, sample and grooves perpendicularto the beam, where the laser first strikes the ESR. ** Bottom reflectoris also on the side walls except for semi-specular bottom films, inwhich case it is ESR on the side walls.

If one or more of the components are graded spatially (e.g., adiffuser), then the overall cavity transport value will be graded in thesame manner. The cavity transport CT can then be determined by averagingthe measured CT values of the cavity over the area.

Applicants have found that for exemplary embodiments of useful backlightcavity geometries, such as a 46 inch diagonal LCD display having acavity depth of 14 mm, CT vales of greater than 0.50 are necessary toprovide a relatively uniform spatial variation of output brightness,when the front reflector R^(f) _(hemi) is 70% or greater and the FWHM ofthe collimate light injection is 60 degrees or less.

Detailed examples are given below which represent a broad range ofreflector types, ranging from substantially Lambertian to substantiallyspecular. The semi-specular examples are for constructions (2) and (3)and were made by covering a specular reflector with a chosen diffuser.All samples were characterized with respect to the angular distributionof reflected light. This was done by using an Autronics Conoscope,available from autronic-MELCHERS GmbH, Germany, in the reflectance mode.The sample is placed about 2 mm from the conoscope lens, at the focalpoint. The sample is illuminated by the instrument with white collimatedlight with a chosen angle of incidence. The light reflected from thesample is collected by the conoscope lens and imaged onto a twodimensional detector array (CCD camera). This image is transformed intoan angular distribution function using the calibration file. Theinstrument provides a very useful comparison of the angular reflectionproperties of various semi-specular and diffuse reflectors. Asignificant specular component of a reflector can result in saturationof the detector near the specular angle, but this value can be measuredseparately on a machine setting of lower sensitivity.

Example F Bead Coated ESR

A film substantially identical to Vikuiti™ ESR film was coated with PMMAbeads mixed with a polymeric binder, similar to the construction ofbeaded gain diffuser films which are commonly used as brightnessenhancement films in LCD backlights. A sample of the film was insertedinto the Autronics Conoscope and illuminated with collimated lightincident at various angles of incidence θ in a plane of incidence whoseazimuthal direction Phi (a rotation of the plane of incidence pivotedabout the surface normal direction) was 0. The measured reflected lightintensity vs. angle data for all theta and phi angles can be visualizedwith a contour plot such as the one in FIG. 12, which is for an angle ofincidence of 45 degrees. The contour plot is a polar plot, with angle ofreflection ranging from 0 degrees to 80 degrees along any azimuthal(Phi) direction. The horizontal direction is referred to as the Phi=0axis, and the vertical direction is referred to as the Phi=90 axis. Thedetector is blocked for an angular range from about theta=42 degrees to80 degrees near the Phi=0 degrees axis, resulting in an artifact as canbe seen in the figure. The angular center of the specularly reflectedcomponent is apparent near −45 degrees along the Phi=0 degrees axis.

As explained above, the degree of specular vs. Lambertian characteristiccan be effectively characterized by comparing the fluxes of the forwardand back-scattered light components (F and B respectively), which can beobtained from the integrated intensities on the left and right halves ofthe plot (integrated intensity on the right and the left of the Phi=90axis). The degree of specularity is then characterized by the Transportratio T=(F−B)/(F+B).

The contour plot of FIG. 12 is for illustrative purposes only and wasnot used in the calculation of T. Instead, quantitative values ofreflectance, shown in FIGS. 13 a and 13 b for the Phi=0 and Phi=90directions respectively, were used. Along the Phi=0 axis, one can seecharacteristics of a Gaussian scattering distribution as well as abaseline Lambertian component which is constant with angle. Data fromθ=42 degrees to 90 degrees is not recorded due to blockage by theincident beam optical components of the Autronics instrument. Forreflectors with slowly varying luminance data at these angles, the datacan be estimated in this blocked area using values in adjacent areas.Along the Phi=90 axis the intensity is relatively flat, as for aLambertian reflector. Integration over all solid angles yields aTransport ratio value of this sample (for a 45 degree incidence angle)of T=0.50.

As explained above, the transport ratio will generally be a function ofangle of incidence. At normal incidence, for most samples, the forwardand reverse components will generally be equal, yielding T=0. However,at higher angles of incidence the degree of forward scattering of agiven reflector will become more apparent. The bead coated ESR wasmeasured at various angles of incidence and the transport ratio T isplotted as curve F in FIG. 14 for these angles of incidence. The maximumtransport ratio occurs near θ=45 degrees. Similarly, the transport ratioof MCPET (see Example C), described further below, was measured in thesame manner. The measured transport ratios are not exactly zero atnormal incidence for these samples, possibly because of small errors inthe measured angle of incidence.

In the plot of FIG. 14, an ideal Lambertian reflector or transmitter(diffuser) is shown in a broken line, and has a value of T=0 for allincidence angles. In contrast, an ideal specular reflector ortransmitter (shown in a thick solid line) has a value of T=1 for allincidence angles except for an incidence angle of exactly 0 degrees, atwhich T drops to 0.

A variety of other reflectors were also constructed as outlined in theadditional examples below. Although the other types of reflectors havediffering reflection properties, the trends are similar. Above 15degrees, the transport ratio generally increases slowly with angle ofincidence. The most rapid increase is at small angles above normalincidence. A pure specular reflector has T=0 at normal incidence and T=1at all other angles. A pure Lambertian reflector has T=0 at all angles.The Examples A through K clearly show that a wide range of transportratios can be made with different reflector constructions. The data issummarized in FIG. 14, and the sample description, labeling, andreflectivity and transport ratio (T) at 45 degrees is given in thefollowing table:

Label Sample description Reflectivity T at 45° A TIPS (0.55 mm) 0.9850.011 B Mitsubishi W270 (125 microns) 0.945 0.065 C MCPET 0.98 0.129 DAstra DR85C + X-ESR 0.965 0.194 F Beaded ESR 0.98 0.496 G PEN/PMMAblend + X-ESR (X-axis) 0.98 0.537 E TiO2/THV + X-ESR 0.97 0.545 HPEN/PMMA blend + X-ESR (Y-axis) 0.97 0.612 I DFA + X-ESR 0.95 0.651 JLenslets on X-ESR 0.99 0.810 K Keiwa PBS070 + X-ESR 0.975 0.943

From a review of these examples and FIG. 14, we characterize a reflectoror other component as semi-specular if: (1) the transport ratio T isgreater than 0.15 (15%), preferably greater than 0.20 (20%), at a 15degree incidence angle, to distinguish over substantially Lambertiancomponents, and (2) the transport ratio T is less than 0.95 (95%),preferably less than 0.90 (90%), at a 45 degree incidence angle, todistinguish over substantially specular components.

Alternatively, one may wish to characterize “semi-specular” as having atransport ratio T greater than 0.2 (20%) at 45 degrees, to distinguishover Lambertian components. To distinguish over specular components, onemay then add the requirement that at least 10% of the light is scatteredin directions which lie outside of a cone of 10 degrees included-anglecentered on the 45 degree specular direction.

Measurement conditions may have to be modified when characterizingreflectors that are locally specular, but are globally semi-specular,such as, e.g., a corrugated or thermoformed thin film mirror. If thespot size of the measurement system such as the Autronics instrument issmaller than the average shape dimensions on the reflector, then severalmeasurements should be taken at different locations on the shapedreflector in order to provide a good estimate of its reflecting angledistribution.

Example E TiO2 Particles in THV on X-ESR

An example of construction (2), a partial Lambertian plus highreflectance specular reflector, was made by laminating a diffusing filmto a broadband multilayer mirror. The mirror was constructed with amultilayer of PEN and PMMA as for conventional Vikuiti™ ESR, but with anextended reflection band (hence the designation X-ESR) that extends from400 nm all the way to 1600 nm. The diffuse film was made by blending0.1% by weight of a white TiO₂ pigment into THV, which was then extrudedand cast as a smooth film. Standard polymer extrusion and film castingprocesses were employed. The small particle size of the titania, coupledwith the large index difference between the titania and the THV (2.4 vs.1.35) results in a wide angle scattering diffuser. Only the lowconcentration of the titania prevents the diffuse film from reflectingmost of the light. Substantial amounts of light pass through the THVfilm and are specularly reflected by the multilayer mirror. Thereflection result, shown in FIGS. 15 a and 15 b for a specular beamhaving an angle of incidence of 45 degrees, shows a combination of anear specular reflection at −45 degrees and a broad Lambertianbackground.

For the Phi=0 trace, the peak value of the specular beam is not shownhere. On a less sensitive recording scale, the measured peak luminanceat θ=−45 degrees was 1,907. The relative strength of the specular andLambertian components can be adjusted by changing the concentration ofthe TiO2 or the thickness of the THV film, or both.

Results similar to those of Example K (below) would be expected forconstruction 1, which involves combinations of a partially reflectingspecular mirror over a high reflectance Lambertian reflector. In thatcase, the relative amounts of specular vs. Lambertian scattered lightcan be adjusted by using partial reflectors with differing transmissionvalues. The Lambertian reflector should remain very high reflectance toavoid transmission losses through the back reflector.

Example K Keiwa PBS-070 on X-ESR

A substantially forward scattering reflector can be obtained bycombining a forward scattering diffuser with a specular reflector. Thisis an example of construction (3). A commercially available diffuser,Keiwa Opulus PBS-070, was laminated to an extended band multilayerspecular mirror (X-ESR) described above. The measured reflectiondistribution, shown in FIG. 16 for the case of 45 degrees angle ofincidence, is close to a Gaussian distribution about the speculardirection. In this case most of the light incident at 45 degrees isreflected in a relatively narrow cone about the specular direction of−45 degrees. The peak luminance along the Phi=90 axis was only 0.8. Thissample has a very high transport ratio at 45 degrees, with T=0.943.

Example I DFA on X-ESR

A broader distribution of scattered light than Example K can be obtainedwith other substantially forward scattering diffusers. Diffusers madewith spherical particles in a matrix that has an index of refractiononly slightly different than the index of the particles can have thisproperty. One example is a diffuser film known as DFA from 3M Co. Theparticle loading and diffuser thickness then determine the degree ofspecularity, which affects the transport ratio. A 0.4 mm thick sheet ofDFA was laminated to the X-ESR and measured in the Autronics instrumentwith 45 degree incident light. Luminance data extracted for reflectanceangles along the phi=0 and phi=90 degree axis are shown in FIGS. 17 aand 17 b. Although there is no pure Lambertian component, the measuredtransport ratio at 45 degrees drops to T=0.651.

Examples G and H Reflector with Asymmetric Transport Ratio

A mixture of PEN and PMMA, with 27% by weight of PMMA, was blended in atwin screw extruder and cast with standard film making equipment. Thecast web was sequentially oriented in a length orienter and in a tenterwith conditions used for making PEN films. The stretch ratios were3.7×3.7. The resultant film was 50 microns thick and had a measuredhemispheric reflectance at normal incidence of about 75%. This film waslaminated to the X-ESR and measured in the Autronics conoscope in twodifferent orientations. Even though the blend diffuser film was orientedequally in the X and Y directions, it exhibits an asymmetric scatteringdistribution. This may be due to elongation of the PMMA disperse phaseparticles during extrusion through the die. The conoscopic plots, shownin FIGS. 18 a and 18 b, illustrate this asymmetry for light incidentalong the X and Y axis respectively. The measured transport ratios at 45degrees were 0.537 and 0.612 respectively. This asymmetry can offer moreoptions for LED placement in LCD backlight designs.

Example A TIPS near-Lambertian Reflector

Various near-Lambertian diffuse reflectors are described in U.S. Pat.No. 5,976,686 (Kaytor et. al.). A particularly efficient reflector canbe made with the process described as thermally induced phase separation(TIPS). A reflector of thickness about 0.55 mm, made by laminating twoTIPS films of thickness about 0.27 mm was measured in the Autronicsconoscope as for the previous examples. The traces along the phi=0 andphi=90 directions are plotted in FIGS. 19 a and 19 b and show anear-Lambertian character. Only a small specular peak is evident at −45degrees. The transport ratio for this reflector was the lowest of allsamples with T=0.011 at 45 degrees.

Example C MCPET

A sheet of diffuse reflector called MCPET, made by Furukawa in Japan,was measured in the Autronics instrument. The reflector was 0.93 mmthick with a hemispherical reflectance of about 98% for normallyincident light. The MCPET has a slightly higher specular component thanthe TIPS film, but is still substantially Lambertian as is evident fromthe phi=0 and phi=90 traces of the conoscopic data shown in FIGS. 20 aand 20 b. A plot of the transport ratio vs. angle of incidence is shownabove in FIG. 14.

The hemispherical reflectance of several other semi-specular reflectorswas measured in the same manner as the Examples above. A shortdescription of each reflector is given below, and the transport ratio ofall measured reflectors is listed in Table II for the case of 45 degreesangle of incidence. The hemi-spherical reflectance of all samples fornear-normal incident light was measured in a Perkin Elmer Lambda 950using a NIST calibrated reference reflector. All reflectors show areflectance of about 95% or greater.

Example D Bulk Diffuser on X-ESR

Astra 85C is a semi-specular bulk diffuser with 85% transmission fromAstra Products. A piece of this diffuser, Clarex-DR IIIC light diffusionfilm, grade number 85C, was laminated to the X-ESR film.

Example B White Reflector

W270, a 125 micron thick white reflector from Mitsubishi, Japan.

Example J Microlens Array on X-ESR

A semi-specular reflector was constructed by casting an array ofmicrolenses on the surface of a film of X-ESR. The microlenses have anoutside diameter of 30 microns and a height of about 6.3 microns. Thelens curvature is spherical, with a radius of 18.7 microns. Themicrolenses were cast in a hexagonal array with about 90% surface areacoverage of the mirror. The lens material was a UV curable resin withindex of about n=1.5. Lenslet of other geometrical shapes and sizes arealso possible.

Backlight Examples

Numerous hollow recycling cavity backlights of different sizes andshapes were constructed using a variety of low loss reflective films andsemi-specular components. In some, for example, beaded-ESR film (ExampleF above) was used as a back reflector, with different asymmetricreflective films (ARFs) used as front reflectors. In others, a gaindiffusing film was included at the front of the recycling cavity with anasymmetric reflective film. Many of the backlights also included lightsource members (e.g., rows of multicolored LEDs disposed in awedge-shaped reflectors) that confined the light injected into thecavity into a full angle-width at half maximum power that issubstantially smaller than a Lambertian distribution. Many of thebacklights exhibited satisfactory overall brightness and uniformity,such as would be suitable for LCD display applications or otherapplications. These backlight examples are described in the followinggroup of commonly assigned PCT Patent Applications, and are incorporatedherein by reference: BACKLIGHT AND DISPLAY SYSTEM USING SAME (AttorneyDocket No. 63274WO004); THIN HOLLOW BACKLIGHTS WITH BENEFICIAL DESIGNCHARACTERISTICS (Attorney Docket No. 63031WO003); WHITE LIGHT BACKLIGHTSAND THE LIKE WITH EFFICIENT UTILIZATION OF COLORED LED SOURCES (AttorneyDocket No. 63033WO004); and COLLIMATING LIGHT INJECTORS FOR EDGE-LITBACKLIGHTS (Attorney Docket No. 63034WO004). Unless otherwise indicated,references to “backlights” are also intended to apply to other extendedarea lighting devices that provide nominally uniform illumination intheir intended application. Such other devices may provide eitherpolarized or unpolarized outputs. Examples include light boxes, signs,channel letters, and general illumination devices designed for indoor(e.g., home or office) or outdoor use, sometimes referred to as“luminaires.” Note also that edge-lit devices can be configured to emitlight out of both opposed major surfaces—i.e., both out of the “frontreflector” and “back reflector” referred to above—in which case both thefront and back reflectors are partially transmissive. Such a device canilluminate two independent LCD panels or other graphic members placed onopposite sides of the backlight. In that case, the front and backreflectors may be of the same or similar construction.

The term “LED” refers to a diode that emits light, whether visible,ultraviolet, or infrared. It includes incoherent encased or encapsulatedsemiconductor devices marketed as “LEDs,” whether of the conventional orsuper radiant variety. If the LED emits non-visible light such asultraviolet light, and in some cases where it emits visible light, it ispackaged to include a phosphor (or it may illuminate a remotely disposedphosphor) to convert short wavelength light to longer wavelength visiblelight, in some cases yielding a device that emits white light. An “LEDdie” is an LED in its most basic form, i.e., in the form of anindividual component or chip made by semiconductor processingprocedures. The component or chip can include electrical contactssuitable for application of power to energize the device. The individuallayers and other functional elements of the component or chip aretypically formed on the wafer scale, and the finished wafer can then bediced into individual piece parts to yield a multiplicity of LED dies.An LED may also include a cup-shaped reflector or other reflectivesubstrate, encapsulating material formed into a simple dome-shaped lensor any other known shape or structure, extractor(s), and other packagingelements, which elements may be used to produce a forward-emitting,side-emitting, or other desired light output distribution.

Unless otherwise indicated, references to LEDs are also intended toapply to other sources capable of emitting bright light, whether coloredor white, and whether polarized or unpolarized, in a small emittingarea. Examples include semiconductor laser devices, and sources thatutilize solid state laser pumping.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe foregoing specification and attached claims are approximations thatcan vary depending upon the desired properties sought to be obtained bythose skilled in the art utilizing the teachings disclosed herein.

Various modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this disclosure, and it should be understood that thisdisclosure is not limited to the illustrative embodiments set forthherein. All U.S. patents, patent application publications, unpublishedpatent applications, and other patent and non-patent documents referredto herein are incorporated by reference in their entireties, except tothe extent any subject matter therein directly contradicts the foregoingdisclosure.

1. A backlight, comprising: a front and back reflector forming a hollowlight recycling cavity, the front reflector being partially transmissiveto provide an output illumination area; a component that provides thecavity with a desired balance of specular and diffuse characteristics,the component being characterized by a transport ratio greater than 15%at a 15 degree incidence angle and less than 95% at a 45 degreeincidence angle, wherein the front or back reflectors can be or includethe component, or the component can be distinct from the front and backreflectors; and one or more light source members disposed to emit lightinto the light recycling cavity over a limited angular distribution;wherein the front reflector has a hemispherical reflectivity forunpolarized visible light of R^(f) _(hemi), and the back reflector has ahemispherical reflectivity for unpolarized visible light of R^(b)_(hemi), and R^(f) _(hemi)*R^(b) _(hemi) is at least 0.70.
 2. Thebacklight of claim 1, wherein the transport ratio is greater than 20% ata 15 degree incidence angle.
 3. The backlight of claim 1, wherein thetransport ratio is less than 90% at a 45 degree incidence angle.
 4. Thebacklight of claim 1, wherein the transport ratio for the component, forlight of a given incidence angle, equals (F−B)/(F+B), where F is theamount of light scattered into forward directions upon interaction ofthe incident light with the component, and B is the amount of lightscattered into backwards directions upon interaction of the incidentlight with the component.
 5. The backlight of claim 1, wherein theoutput illumination area defines a transverse plane, and the lightsource members emit light into the light recycling cavity with a fullangle-width at half maximum power (FWHM) relative to the transverseplane in a range from 0 to 60 degrees.
 6. The backlight of claim 5,wherein the light source members emit light into the light recyclingcavity with an FWHM relative to the transverse plane in a range from 0to 30 degrees.
 7. The backlight of claim 1, wherein the light sourcemembers include one or more LEDs.
 8. The backlight of claim 1, whereinthe front reflector has a reflectivity that generally increases withangle of incidence, and a transmission that generally decreases withangle of incidence.
 9. The backlight of claim 8, where the reflectivityand transmission of the front reflector are for unpolarized visiblelight in any plane of incidence.
 10. The backlight of claim 8, whereinthe reflectivity and transmission of the front reflector are for lightof a useable polarization state incident in a plane for which obliquelight of the useable polarization state is p-polarized.
 11. Thebacklight of claim 1, wherein R^(f) _(hemi)*R^(b) _(hemi) is at least0.75.
 12. The backlight of claim 11, wherein R^(f) _(hemi)*R^(b) _(hemi)is at least 0.80.
 13. A hollow light recycling cavity comprising a frontand back reflector, the front reflector being partially transmissive toprovide an output illumination area, wherein the cavity comprises acavity transport value of greater than about 0.5 and less than about0.95, and further wherein the front reflector comprises an R_(hemi) ofgreater than about 0.7.
 14. The cavity of claim 13, further comprising acomponent that provides the cavity with a desired balance of specularand diffuse characteristics, wherein the front or back reflectors can beor include the component, or the component can be distinct from thefront and back reflectors.